The present invention relates to improvements in the design and operation of wireless remote control systems, and is particularly, although not exclusively, concerned with telemetry systems, whereby sensors are controlled and monitored remotely so as to monitor conditions in a harsh environment such as, for example, an engine or a gearbox. The applicant's own paper, “A Digital Electronic Solution to Piston Telemetry”, SAE Technical paper series no. 2000-01-2032, discloses a prior telemetry system for use in a piston.
Wireless piston telemetry in which a transceiver mounted within a piston transmits signals from one or more sensors to a base station, situated outside the hostile environment of the piston, has provided a more versatile and more useful means of monitoring the conditions to which pistons are subjected, than have prior, hard-wired systems, which involve complicated electrical connections between the sensor(s) and the processing unit (base station) which is typically mounted externally of the piston and, usually, on the wall of the crankcase. However, even the prior art wireless systems are subject to problems.
For example, the piston circuitry must have a power source in order to transmit signals from the sensor to the processing unit. A known prior art system uses a battery, located in situ with the sensors and their associated circuitry, including a transceiver, to supply the circuitry with power. In order to supply power to the circuitry a switch must be closed. However, this has to be done before the piston is installed in the engine, and once the piston is installed the switch can only be accessed by dismantling the engine. Because of this the power remains “on” and the battery continues to discharge for as long as the piston is in the engine. Furthermore, the average lifespan of the conventional batteries is approximately 4 hours, and standard engine testing techniques require engines to run continuously for up to 100 hours.
To overcome the problem of the lifetime of the battery, power generation systems have been proposed which aim to recharge the battery mounted within the piston. However, those currently available require the piston to be modified and/or cause other components to perform in such a way that the motion of the piston may be influenced. This causes the engine to function other than as intended, and possibly increases the temperature inside the piston, which has an adverse effect on the circuitry inside the piston. Therefore any results obtained may be subject to errors.
A further problem with prior wireless telemetry systems is that even though the battery may be charged when the engine is running, as soon as the engine is switched off the battery begins to discharge, since the circuit is still operational and drawing a current. If the battery discharges fully before the engine is switched on again, there will be no energy in the battery to perform pre-runtime checks.
Turning on the engine would cause the battery to re-charge, and, provided the circuit could be turned on, measurements could then be taken, as the circuit would be fully operational. However, pre-runtime checks, with an inactive engine, would be impossible. It would therefore be desirable to be able to turn off the circuit to conserve charge, and to turn it on again at a chosen time. However, if the circuit is in an off state, it would not ordinarily be receptive to an instruction to turn on.
A further, related problem arises if the telemetry system is arranged for continuous transmission of data—i.e. one-way communication—which is sometimes preferred. The circuit may be transmitting data continuously from a single sensor or else from a plurality of sensors which are polled sequentially. In either case the operator cannot change the operational mode of the circuit, either to alter the sequence or to turn off the circuit, because the circuit is transmitting only—i.e. it is “talking” but not “listening”. One possible solution to this is to arrange for the circuit to cease transmission periodically and to “listen” for an instruction in its quiet periods. This is not ideal, as the circuit must repeatedly break transmission in case an instruction is being sent. Clearly it is desirable to provide a means of conveying an instruction to a circuit whether or not it is currently receptive.
Another problem facing known piston telemetry systems is that of the loss of communication which occurs when the temperature of the engine rises. This is due to the fact that the frequency of the carrier wave, conveying information between the sensors and the processing unit, drifts as a function of the temperature of the circuit producing the carrier wave. This means that communication between the piston circuitry and the processing unit becomes broken, as the signal can no longer be detected by the processing unit.
Embodiments of the present invention aim to address at least partly the above mentioned problems.